Originally Posted by Fyutga
Looks like you have the common wet-cell type battery. Have you ever checked the electrolyte level within each cell in the previous two years you've had the batteries?
If not, it's certain that the explosion was caused by low electrolyte level.
The electrolyte is normally lost over time and repeated charging cycles through outgassing which occurs as part of the chemical reaction when the charging electrons flow through the electrolyte. As electrolyte levels in the cells drops, more space is created above the cells for the accumulation of the hydrogen gas created by the outgassing.
When electrolyte levels drop low enough to expose the plates that make up each cell the electrical charging current may arc across the plates since there is now not enough electrolyte to carry the current. In the presence of accumulated hydrogen - like in the Hindenberg - the arc will ignite the hydrogen and cause the explosion you experienced.
You must check electrolyte level and top off the "water" periodically to avoid this situation.
If you have been topping off, and are certain there was sufficient level of electrolyte in the battery prior to starting the charge, then it may be that the charger went haywire, overcharged the battery, and boiled off the electrolyte, leading to the above scenario. A quick and dirty check would be to connect the charger to a fully charged battery and measure its output voltage.